One example of biological wastewater treatment as commonly practiced is illustrated schematically at 20 in FIG. 1, which is labeled “Prior Art.” FIG. 1 is intended to be representative, not exhaustive, as there are many different ways to set up a plant for biological wastewater treatment. FIG. 1 is described immediately below to provide a context for the invention.
Influent raw wastewater 22 from a source thereof, including for example, a municipal sewage system, undergoes a mechanical separation in a process unit 23 called the “head works,” typically by bar screening and grit removal. Bar screening removes larger foreign objects including, for example trash and tree limbs and the like that may be swept along by the sewage as it is collected for delivery to the treatment plant. Grit is removed to keep fine particles from accumulating in the plant and clogging pipes and pumps. Raw wastewater may also be treated in a primary sedimentation tank, normally after the head works and not illustrated here, for a period of quiescent settling to allow greases, oils, and floating objects to be skimmed from the top as scum and to remove matter from the bottom that readily settles prior to entering reactor 26 for biological nutrient removal (BNR″). The screened wastewater 24 having grit removed and possibly having undergone primary sedimentation enters reactor 26 where the wastewater is mixed with an activated sludge for BNR treatment. The sludge comprises microbes that digest the organic carbonaceous content of the wastewater in the BNR process, often in at least one or more of a series of reactions characterized by the nature of the respiration of the microbes. Aerobic respiration occurs in the presence of dissolved oxygen, in which air as a source of oxygen typically is transferred through the mixture of sludge and wastewater. Aerobic conditions are also commonly referred to as “oxic” conditions. Anoxic respiration occurs when the dissolved oxygen has been consumed and only chemically bound oxygen remains. Anaerobic respiration occurs when there is no dissolved oxygen available and the chemically bound oxygen has been consumed. Many processes have been developed around these respiration conditions for biological nutrient removal in the activated sludge process.
For example, early BNR processes used only an aerobic BNR reactor. Later processes added an initial anoxic phase. The well-known and widely used Bardenpho process uses a four-stage sequence of anoxic, aerobic, anoxic, and aerobic reactions to treat wastewater and for nitrogen removal. The basic Bardenpho process was later modified to add an initial anaerobic zone as the first of five stages in what is commonly practiced today as a modified Bardenpho BNR process. Subsequent developments include a sequence of anaerobic and aerobic zones in which an anoxic zone is introduced. The initial anaerobic zone is considered generally to improve biological phosphorous removal in a BNR reactor.
Once biological nutrient removal has occurred, the mixture of wastewater and treated sludge 28 typically enters a clarifier 30 for gravity separation of the liquid fraction from the solids fraction. Following clarification, the clarifier releases the resulting liquid fraction 32, called “supernatant,” from the top of the clarifier for additional treatment prior to discharge. The clarifier discharges the activated sludge 34 from the bottom of the clarifier.
For further treatment, the clarified wastewater normally is filtered as illustrated at 36 to remove suspended solids. The filter backwash, 38, which comprises the filtered suspended solids, is returned to join the municipal or other source of wastewater and to re-enter the head works 23 in influent steam 22. Clarified and now filtered wastewater 40 may undergo ultraviolet (“UV”) radiation treatment 42 to reduce or eliminate contaminating microbes. Effluent treated wastewater 44 is removed from UV treatment unit 42. If the effluent treated wastewater meets standards for treatment, usually set out by operation of law, then the approved effluent 44 may be disposed of in fresh water lakes, used for irrigation, or other permitted purposes, including ultimately participating in the water cycle, further treatment for use as potable water, and perhaps to return another day to the influent 22 for biological nutrient removal. If the effluent treated wastewater 44 does not meet standards for treatment, then rejected effluent 46 can be diverted to a reject holding pond 48, ultimately to be returned as reject return flow 50 to join the municipal or other source of wastewater and to re-enter the head works 23 in influent steam 22.
Returning now to the clarifier 30, the solids fraction 34, comprising activated sludge, is normally taken from the bottom of the clarifier. The settled activated sludge stream 34 is separated into two streams, a return activated sludge stream (“RAS” stream) 52 for seeding the BNR reactor and the remainder of the sludge for disposal, a waste activated sludge stream (“WAS” stream) 54. Most plants generate some degree of waste activated sludge. When WAS 54 is obtained, which is the usual case for many biological wastewater treatment plants, then it typically is disposed of in one of two ways. One option is treat the WAS inline at the plant in a digester 56. Another option is simply to haul the WAS stream 54 offsite for further treatment or disposal in a land fill, as illustrated at 57. Offsite treatment or disposal is illustrated in dashed lines to indicate it is an option that may be practiced at some facilities. In some offsite treatments, the WAS stream 54 is treated by heat drying or chemical addition to produce Class AA solids for commercial purposes.
Untreated WAS and even digested activated sludge that otherwise does not meet minimum standards for stabilization cannot be used as fertilizer or otherwise disposed of in surface applications and is often buried in a landfill. If the sludge is to be disposed to a landfill rather than in a surface application, then often about the only requirement imposed is that the sludge pass a minimum dryness check to determine whether the WAS is sufficiently dewatered for disposal.
Additional benefits of hauling the WAS stream 54 away from the plant without inline downstream biosolids treatment is that liquid side streams produced by inline treatment are not returned to the influent to the plant and do not add to the nutrient loads treated in the BNR reactor 26. Side streams typically contain nitrogen from the production and breakdown products of ammonia, including ammonium, nitrates and nitrites; phosphorous in several soluble phosphate forms in the side stream liquid, including primarily reactive orthophosphate, condensed phosphates, including pyro, meta, and polyphosphates, and organic phosphates; and carbonaceous material, which material produces a demand for oxygen for respiration by microorganisms in the biological wastewater treatment process and is often called “biological oxygen demand” (“BOD” or “CBOD”). High rates of these substances in side streams can upset the balance of the ecological system in the BNR reactor, resulting in less efficient operation, difficulty meeting effluent permits, increased oxygen demands, increased additions of chemicals to remove nutrients not removed by the microbes, and increased costs of operation. Nevertheless, although numerous benefits readily appear for a biological waste water treatment plant in hauling excess activated sludge away from the plant instead of treating it inline, transport and disposal costs for WAS taken offsite offset perceived benefits in many cases.
Transport and disposal costs can be reduced by treating, dewatering, and recovering stabilized biosolids prior to transport in a process known as “sludge stabilization,” which is commonly practiced. By “stabilizing” the biosolids, we mean at least the reduction of the organic content, which typically also reduces the concentration of pathogenic bacteria, the potential for sludge odor, and provides uniformity of sludge characteristics, including moisture content, in the recovered and stabilized biosolids.
If the stabilized biosolids are to be applied to land in the United States, as opposed to disposal in a landfill, then the biosolids are subject to regulations promulgated by the United States Environmental Protection Agency as set forth in Volume 40 of the Code of Federal Regulations, at Section 503, which are often referred to as “Class B biosolids standards.” Class B biosolids standards require a minimum reduction in “vector attraction” and “pathogens.” Pathogen reduction means that the biosolids are without significant coliform pathogens as defined by the federal regulations. Pathogen reduction is normally determined by the fecal coliform test. Reducing vector attraction means that the biosolids are not attractive to rodents and mosquitoes and the like vectors for disease transmission. Determining the degree of reduction in vector attraction includes ten (10) to twelve (12) different methods. One commonly used test for determining vector attraction reduction measures a parameter called “specific oxygen uptake reduction,” known by the acronym “SOUR.” Other countries or municipalities outside the United States may impose similar or related minimum requirements for surface disposal of stabilized biosolids and individual local municipalities may impose more stringent conditions. Some sites require a minimum biosolids content to satisfy local stringent regulations for dryness.
One of the more efficient current methods of sludge disposal, illustrated in FIG. 1, is to treat the WAS 54 downstream of the clarifier 30 and inline in a digester 56 at the biological wastewater treatment plant 20 where it originated and to dewater the digested sludge in a mechanical dewatering unit 58 for recovery of the stabilized biosolids 59. In plants incorporating sedimentation tanks for the raw sludge influent to the plant, called “primary sedimentation tanks,” the raw primary sludge may be drawn from the tank and added to the WAS for treatment in the digester, which is not illustrated in FIG. 1.
The two primary solids stabilization systems are typically either anaerobic or aerobic digesters, although combinations of these two primary systems have been proposed. An aerobic digester is illustrated at 56. Hydrolysis of the WAS in the digester reduces the organic content of the sludge and the concentration of pathogens. The digested WAS 57 is pumped to a mechanical dewatering unit 58 to remove water from the sludge and recover the biosolids. The separated liquid fraction 60 from both anaerobic and aerobic digesters is normally returned to the influent to the plant from the dewatering unit as a side stream, and often results in nutrients, including nitrogen, phosphorous, and even sometimes CBOD being added to the source influent waste water 22 to enter the head works 23 and BNR reactor 26. Side streams 60 from aerobic digesters typically return phosphorous and nitrates, but not ammonium and CBOD due to aeration; side streams from anaerobic digesters typically return ammonium, CBOD, and phosphorous. It is beneficial for efficient nutrient removal in BNR reactor operation to produce a digester side stream for return to the influent that is low in phosphorous, ammonium and other forms of nitrogen, and CBOD. Return of the side stream can place additional demands on the BNR reactor and can overload the system so that additional treatments to remove nutrients have become somewhat routine and increase the cost of biological nutrient removal. Decanting the digester can exacerbate this problem. In decanting, the WAS is allowed to settle in the digester and the gravity-settled WAS provides a supernatant liquid fraction 61. The decanted supernatant liquid may be returned to the influent to the plant as a side stream or added to the side stream 60 and returned, as is illustrated in FIG. 1.
If the WAS 54 is treated in-line after clarification in an aerobic digester 56, then aeration devices including aerators and blowers and mechanical mixing devices 62 may typically run subject to control based on time, monitored dissolved oxygen (“DO”), monitored oxidation-reduction potential (“ORP”), or a combination of one or more of these. Alternatively, and somewhat commonly, the aeration and mechanical mixing devices may run manually in an uncontrolled fashion for from about 10 to 24 hours per day. After digestion, flocculating agent 63 is added inline to the digested biosolids 57 and the now-flocculated and digested bio-solids 57 are mechanically dewatered at 58.
Despite years of development, biological phosphorous removal still remains problematic. Conventional aerobic digester practice, including monitoring DO concentration, ORP, time, and using manual aeration control tends to lower the pH in the digester biosolids and to produce high orthophosphate phosphorous and nitrate nitrogen levels in the digester side streams. Fully 70% of the energy costs associated with operating a biological waste water treatment plant can be attributed to aeration, and blowers or surface aerators in the digesters are a significant source of these costs.
The relatively high energy requirement associated with oxygen transfer by adding air or another oxygen source and mixing the sludge to dissolve the air is the primary disadvantage typically attributed to the aerobic digestion process. Subsequent developments to reduce energy costs have included fine bubble air diffusion to increase oxygen transfer efficiency and increased temperatures for thermophilic operation, but each of these developments introduces additional issues. Another disadvantage in aerobic digestion of WAS is the high cost of chemical additions due to the soluble phosphorous and nitrate nitrogen concentrations in the digester side stream returned to the influent waste water. High nitrogen levels in the side stream produce yet additional challenges for nitrogen removal efficiency in the BNR reactor.
Excess phosphorous concentrations in the BNR, whether from aerobic or anaerobic digestion, typically result from the continual return of phosphorous to the BNR influent through digester side stream 60, typically predominated in the species of reactive orthophosphate PO43−. Orthophosphate returned to the influent from the digester side stream tends to accumulate and eventually overloads the plant. Chemicals normally are added to the BNR reactor at the beginning or end of the reaction sequence to bind and precipitate the excess phosphorous so that it can be removed during sludge wasting with the stabilized biosolids. Chemical additions to the BNR reactor of aluminum sulfate, ferric sulfate, ferric chloride, or, somewhat less commonly, lime and the like chemicals, remove excess orthophosphate phosphorous, but also increase plant operating expenses.
It would be desirable to improve systems for biological nutrient removal in wastewater treatment, and especially to increase the efficiencies of the aerobic digester processes used to treat waste activated sludge.